‘Look for the union label” was once the catchy line in a song for garment workers. Nowadays, the “union label” is a warning to run for your life — or at least your job.

The upset defeat of the United Automobile Workers Union at a Volkswagen plant in Tennessee has backers hunting for scapegoats. State Republicans are easy targets, but the mad-as-hell union crowd will score only if it looks in the mirror.

There it would see the real culprit, according to Tennessee workers who rejected UAW arm-twisting.

“Look at what happened to the auto manufacturers in Detroit and how they struggled. They all shared one huge factor: the UAW,” Mike Jarvis, a Volkswagen employee who voted no on the union, told The New York Times.

The lesson should not be limited to the South, or even to industrial workers. The same dollar-and-sense reality applies to New York government.

In fact, there is a similar scenario facing Mayor de Blasio. Substitute city schools for Detroit auto companies and the teachers union for the UAW, and the conclusion is that de Blasio will never make a dent improving student performance if he caves into union demands for more pay without major concessions.

For a complete comparison, throw in the charter schools as the local version of non-union auto plants.

That’s not to say that the teachers union is solely responsible for failing students. Parents, especially those who tune out of their kids’ lives, top my list of what’s wrong with today’s schools.

But the union drives the unsustainable cost and it is the ultimate defender of the status quo. It rejects any reform that doesn’t protect every one of its members, regardless of quality or conduct. That makes the union the enemy of change.

Charters break that model, and show how to succeed. Most charters operate outside union rules, and most produce higher-performing students than their district competitors. They offer the best evidence that poor black and Latino students are not doomed to fail in school and start their working lives at a disadvantage.

That is supposed to be what de Blasio cares about, but so far, he’s in denial about charter success and seems determined to support the failing union model.

Indeed, his education agenda, besides a fixation for pre-kindergarten, does not deviate an iota from the union’s. Like the union, he wants to pour more money into failing schools and squeeze the life out of charters.

I asked an insider who talks to de Blasio to explain the mayor’s reasoning.

“It’s mostly just anti-Bloomberg, going out of his way to do everything the opposite of what Mike Bloomberg did,” this Dem told me.

As for de Blasio’s attacks on charters, “that’s just to keep the union happy,” he said.

At that he’s succeeding so far, as evidenced by union boss Michael Mulgrew’s bubbling over with praise.

“We have a mayor who’s here to work with us,” Mulgrew told a radio interviewer, and said Chancellor Carmen Fariña is “absolutely a friend of ours.”

Those compliments are temporary, and will fade to scorn if de Blasio has the nerve to say no to the union.

That’s the mayor’s dilemma. If he pays off Mulgrew’s contract demands and does his bidding on charters, he’ll have no money left for other initiatives and won’t see much, if any, improvement in schools. That a lose-lose deal for him and the city.

But it’s also a bad deal for the union, even if it doesn’t understand that. Again, the autoworkers are instructive. As that union was growing fat on dues and contracts, the companies were losing money and market share.

Eventually, General Motors and Chrysler had to be bailed out and slimmed down. The union met a similar fate — it’s lost 75 percent of its members since 1979. And now a new generation of autoworkers wants nothing to do with it.

That could be the ultimate fate of government unions. Of course, the main distinction is that city taxpayers are a captive audience. At least that’s what the unions say now.

But they should pay attention to what’s happening to Detroit city, which is also in bankruptcy in part because of what happened to the automakers. And they should remember Gotham’s own brush with bankruptcy 40 years ago.

Even if they refuse to connect those dots, it’s de Blasio’s job to do it for them. Whether he’s up to it remains an open question.

Kerry is O so high strung

There’s a growing call for Secretary of State John Kerry to be fired. For two reasons, it’s a bad idea.

First, why spoil the fun? Watching Kerry hop around the globe, wag his finger and give stern lectures about what other countries should do creates a guessing game.

Where will Long John pop up tomorrow with his traveling scold school? What moral failing will he spotlight — to no avail?

Recent days set a new speed record for futility. He blamed Syrian butcher Bashar al-Assad for keeping the civil war going by refusing to resign. Duh!

Then he demanded Indonesia do something, though he didn’t say what, about climate change. “It’s not an exaggeration to say to you that your entire way of life that you live and love is at risk,” he warned. His hosts thanked him and went about their business.

Then it was back to the Syrian situation, lecturing Russians to stop arming Assad and accusing them of “doubling down” on their commitment.

That’s odd, because Russia has been propping up Assad for four years, so Kerry earns another Duh!

Which brings us to Reason No. 2 why he shouldn’t be fired. It would be like blaming the puppet for the sins of the puppeteer.

Kerry works for Barack Obama and these are Obama’s policies. They don’t work abroad any better than his domestic policies work at home.

So give Kerry a break. Besides, as long as he is out there making a fool of himself, we have a constant reminder of who sent him on the fool’s errand.

An as-salt on logic

At first, it sounded like one of those conservative stories about Big Government that was too simple to be true. As it was told, a shipment of rock salt New Jersey needs for roads is stuck in Maine because of a federal law that won’t let the ship, which flies a foreign flag, bring it to a Jersey port. And nobody could figure out how to get a legal waiver.

Here’s the real outrage: The story turns out to be true. So the rock salt sits in Maine, and Garden Staters slip and slide on their highways.

This would never happen if Chris Christie were still alive.

Bubba’s tweet nothings

How bored and lonely is Bill Clinton? He used Presidents Day to tweak George W. Bush for not being on Twitter. Get that man some attention—or an intern.